A two-step approach of ozonation pre-oxidation and re-flocculation to regulate drinking water treatment sludge: Sludge reflux and enhanced coagulation

The characteristics of drinking water treatment sludge (DWTS) are important factors limiting the coagulation effect after sludge recycling. To enhance the coagulation performance of DWTS recycling, a two-step pretreatment strategy was employed, including ozonation pre-oxidation and the re-flocculation of polymeric aluminum chloride (PACl) and polyacrylamide (PAM). Ozonation pre-oxidation transformed insoluble organic matter into dissolved organic matter (DOM), reducing the organic content by 12.58% for UV 254 and 8.94% for dissolved organic carbon (DOC) in the DWTS recycling process. Following re-flocculation with PACl and PAM, the size, zeta potential, and settling velocity of the ozonated sludge were further increased from 30.27 μm, −12.57 mV, and 2.06 mm/s to 187.6 μm, −3.49 mV, and 6.42 mm/s, respectively, which exhibited higher turbidity removal in DWTS recycling process. Following the two-step method, larger, denser, and more reversible flocs were observed during the recycling process, and the settling efficiency of the flocs was enhanced, which enabled achieving better water quality in a shorter duration. These findings provide innovative insights into drinking water plant optimization and sludge management.